A toxic spill at a copper mine in the northwestern state of Sonora is the Mexican mining sector’s worst environmental disaster in recent history.

The mine is owned by mining giant Grupo México, Mexico’s largest mining corporation and operated by its Buenavista del Cobre division. Grupo México is the third largest copper producer in the world and has a rail transport division, Ferrocarril Mexicano (Ferromex), that operates Mexico’s largest rail fleet. The Buenavista del Cobre mine, part-way through a $3.4 billion expansion plan, has some of the largest proven copper reserves in the world and is the world’s fourth largest copper mine.

The spill allowed 40,000 cubic meters of toxic copper sulfate acid to enter the Tinajas stream in the town of Cananea on 6 August 2014. Buenavista del Cobre claimed the spill was the result of an unforeseeable heavy rain storm, which triggered a rise in the level of water and copper sulfate in a holding tank being constructed at the copper mine. Grupo México has formed a team of 20 experts from the University of Arizona and Mexican universities to investigate the spill.

However, an initial report by the National Water Commission (Conagua) determined that the spill was caused by a flawed polyethylene pipe at one of the mine’s leachate tanks, together with a faulty valve at another tank. Conagua attributed the environmental disaster to negligence on the part of the company. Mexico’s federal environmental protection agency (Profepa) reported that the contaminants from the spill included copper, arsenic, aluminum, cadmium, chromium, iron, manganese and lead.

Credit: Jesus Ballesteros/Expreso-Cuartoscuro.com

Mexico’s Environment Secretary Juan José Guerra Abud called it the “worst natural disaster provoked by the mining industry in the modern history of Mexico,” and confirmed that the spill contaminated not only the 17.6-kilometer-long (11-mile-long) Tinajas stream, but also the River Bacanuchi (64 kilometers in length), the River Sonora (190 kilometers long) and the El Molinito reservoir which stores 15.4 million cubic meters of water.

The contamination turned the waterways orange (see image) and affected the water supply of 24,000 people in seven communities along the rivers, forcing schools to close for several weeks while environmental authorities clean up the mess. More than 300 wells were shut down. The Sonora state government has been providing millions of liters of water via trucks to residents in the affected area. It has also started a temporary employment program to reactivate the local economy. The mining company has provided 13 million liters of water and $266,000 in immediate assistance to affected communities.

Some 800 mine workers, members of Mexico’s national mining and metallurgical workers union, blockaded the mine entrances in protest at the company’s failure to prevent the spill. Workers have been fighting over contracts since a strike in 2007.

The total clean-up costs are unknown, but likely to run into tens of millions of dollars.

On 18 August, Profepa filed a criminal complaint against Buenavista del Cobre and another Grupo México unit, Minera México, for their alleged roles in the spill. Grupo México could be fined up to 3.3 million dollars if the complaint is upheld.

Sonora State Governor Guillermo Padrés Elías has announced that the seven municipalities affected by the leached copper spill are filing a civil claim for damages.

Eight years ago in the northern state of Coahuila, an explosion in a coal mine belonging to Grupo México left 65 miners trapped underground; only two bodies were ever recovered.